Create delete callback for client-side polling

[jamoore]

Similar to ProcessCallbackI which does not consume a server thread for waiting on scripts to finish, a similar callback is needed in the case of delete. In the initial version, it will probably work strictly by polling the delete handle.

Example usage:

    	DeleteCallbackI cb = new DeleteCallbackI(client, handle);
    	Integer errors = null; 
    	while (errors == null) {
    	    errors = cb.block(500);
    	}

[jmoore]

(In [7850]) First version of client-side callback object for delete queue (Fix #2731)

[jmoore]

(In [7856]) Making callback protected for subclass use (See #2731, #2732)

[jmoore]

(In [8188]) Throwing ObjectNotFoundException immediately from delete callback (See #2731)

[jmoore]

(In [8474]) Also catching ObjectNotFoundException in OmeroCpp and OmeroPy (See #2731, r8188)